The present application is directed to a two-wire processing network which employs a two-wire process control loop to connect field mounted two-wire processing devices to a process controller. More particularly, the present application relates to the provision of passive power conditioning and pseudo isolation to such networks.
Two-wire processing networks have been developed as a way to reduce the wiring required due to the placement of processing devices in the field, remote from the controller.
One class of process devices are process measurement devices which include process variable transmitters, used to measure a process variable such as pressure, temperature, flow or position and communicate the measured variable to a process controller. Another type of process device is an actuator, such as a valve or motor controller or the like. Generally, process control is accomplished using a combination of transmitters, actuators, and a process controller that communicate across a process control network. Both types of process devices interact with the physical process through process interface elements, which are devices that relate electrical signals to physical process conditions, and include devices such as sensors, limit switches, valve controllers, heaters, motor controllers, and a number of other devices.
The process controller may, among other components and systems, be a microcomputer or other intelligent device located in a control room distant from the monitored processes. The process controller can receive process information from one or more process measurement devices and apply a suitable control signal to one or more process control devices to control the process.
The two-wire devices receive power from the process control network, and communicate over the process control network in a manner that is generally unaffected by the provision of power to the process device. Techniques for communicating over two-wires include 4–20 mA signaling in accordance with one of a number of protocols, including those known as the Foundation Fieldbus Protocol and the Profibus Protocol, among others.
Although two-wire process control systems permit wiring simplification, such systems only provide a limited amount of electrical power to connected process devices. For example, a process device that communicates in accordance with 4–20 mA signaling is to draw no more than 4 mA, otherwise the device's current consumption could affect the process variable.
An important aspect of the two-wire network is the need to create appropriately conditioned power, therefore two-wire networks incorporate some form of power conditioning. A power conditioner is a circuit that connects between a power supply and the network connecting the process devices and controller to create the impedance needed to allow communication and power to co-exist on the same pair of wires. Power conditioning circuits used in two-wire networks are commonly broken down into two categories, passive power conditioners and active power conditioners. Requirements and characteristics particularly related to Fieldbus power conditioners, as well as other aspects of the Fieldbus networks are provided in a set of identified standards as detailed in the International Fieldbus Standards Series IEC 61158 and 61784, both hereby incorporated by reference in their entirety. Of particular interest as related to power conditioners is the IEC 61158-2, which provides standards for Fieldbus networks used as industrial control systems, including the physical layer specifications.
While active power conditioners are being used in existing systems, the present disclosure is particularly directed to passive power conditioning. Benefits obtainable by a passive power conditioner include the potential for high reliability due to the absence of active components, and the economic advantage of inexpensive passive components.
In accordance with the IEC 61158-2 power conditioner requirements, an ideal passive power conditioner includes a 5 milli-Henry inductor with a 50 ohm resistor placed in series between the power supply and the network. This design, in conjunction with trunk terminators of the Fieldbus network, creates a critically damped circuit with an impedance ranging from 40 to 60 ohms across a frequency ranging from DC through 100 kHz. A problem with this ideal conditioning (damping) circuit is that use of the 50 ohm resistor results in high voltage loss, particularly in high current demand situations. For example, if the bus current demand for a trunk is 300 milli-amps, then the voltage loss across the 50 ohm resistor would be approximately 15 volts. Commonly, two-wire networks are supplied by a 24 volt source, and a process device may require a minimum of 9 volts for operation. Since there would also be voltage drops in the network due to cable resistance, under such conditions either the trunk cable limit would be driven down to an impractical 0 meters, or there would be insufficient voltage for the processing device. Additionally, the voltage loss generated by the 50 ohm resistor would amount to 4½ watts which is generally unacceptable for today's technological solutions. To address this situation, a user may be tempted to remove the resistor. However, this will result in network instability and communication distortion among other problems.
It is also inferred in the standards set forth in IEC 61158-2 (i.e, clause 12, formerly clause 22) that galvanic isolation should be provided for each segment of a Fieldbus network. As used within this discussion, galvanic isolation is to be understood to mean true electrical isolation between components. Such isolation will lower or remove parasitic cable crosstalk, as well as or in addition to maintaining adequate power source impedance at times when one bus conductor in the system may fail to ground. Parasitic crosstalk may also occur when a bus conductor fault, such as described above, occurs on two discretely conditioned segments of the Fieldbus that do not have galvanic isolation from each other. It is to be understood, however, that due to component parasitic influences and the requirement for adequate common mode and/or differential mode noise suppression, ideal galvanic isolation in existing power conditioners may not be completely obtainable. Existing power supply systems are adequate for dealing with in-band frequency influences and frequency influences below the in-band frequency, in view of the recommended network constraints outlined in IEC 61158-2 at clause 12. In situations where redundant topologies exist, segment isolation presents a cost, size and efficiency penalty which progressively increases with an increase in output power. Also, for existing power supplies, frequencies increasing above the in-band frequencies become proportionally problematic, as will be expanded upon below.
Thus, existing Fieldbus and other two-wire networks suffer from various drawbacks. Among these are inefficiencies and limitations of operation, especially at high current demand levels for networks employing active or ideal power conditioners, as well as inefficiencies in attempting to provide isolation between network segments, particularly when frequencies will increase above an in-band frequency range. The following will discuss solutions to the foregoing problems.